CHAPTER Xt 

 THE SIMPLEST MANY-CELLED ANIMALS 



Cell differentiation and body organization in Hydra. 



From the examination of Hydra we have learned that 

 there are true many-celled animals which are much less 

 complex in structure than the toad and crayfish. The 

 body of Hydra, like the body of the toad, is composed of 

 many cells, but these cells are of only a few different 

 kinds; that is, show but little differentiation. There is 

 relatively little division of the body into distinct organs. 

 Still, certain parts of the body devote themselves princi- 

 pally to certain particular functions. Thus all the food is 

 taken in through the single "mouth-opening" at the 

 apical free end of the cylindrical body, and there are 

 certain organs, the tentacles, whose special business or 

 function it is to find and seize food and to convey it to the 

 mouth. After the food is taken into the cylindrical body- 

 cavity it is digested by special cells which line the cavity. 

 Some of these cells are unusually large, and each contains 

 one or more contractile vacuoles. From the free ends of 

 these cells, the ends which are next to the body-cavity, 

 project pseudopods or flagella. These protoplasmic 

 processes are constantly changing their form and number. 

 In addition to these large sub-amoeboid cells there are, in 

 this inner layer of cells lining the body-cavity, and 

 especially abundant near the base or bottom of the cavity, 

 many long, narrow, granular cells. These are gland- 

 cells which secrete a digestive fluid. The food captured 

 by the tentacles and taken in through the mouth-opening 

 disintegrates in the body-cavity, or digestive cavity as it 



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